(compiled from various data sources, which
can vary!)
M = group 1 or group 2 metal and
X = group 6/16 or group 7/17 halogen
non–metal
The formulae are either MX, M2X, MX2
or MX e.g. NaCl, Na2O, Na2S, MgCl2, MgO,
MgS etc.
What
you might expect and why?
As already described
above for lattice enthalpies of ionic compounds, many melting point
trends can be explained with reference to Coulomb's inverse–square
law relating the attractive force between two electrically charged particles
F
c+ x c–
/ d2
(Note that
d = r+ + r–
(d = total of the two ionic radii)
The smaller the ionic radii and the
greater the charge on the ions, the stronger the ionic bond and the
higher the melting point.
BUT, small ionic cation radii with
a high charge density and/or combined with larger more polarizable anions can
produce varying degrees of covalent character i.e. decrease in ionic character of the
ionic bond.
There are also comparison problems
due to differences in the arrangement of
ions in the crystal lattice.
For cations, the smaller the radius
and the greater the ionic charge, the greater the charge density –
the greater its attractive force towards a negative anion of an
ionic bond and increasing the melting point – greater KE needed to
break the ionic bond ...
BUT, these two factors also
increase the ability of the cation to polarise an anion and
create covalent character in the bond, which may lead to a
decrease in melting point compared to a purely ionic structure.
Typical melting point trends, but not always consistent
Group 1 chlorides from LiCl to
LiI, decrease in melting point down the group, but LiF anomalous.
In fact few trends are
completely
straight forward, quite unlike the much more consistent trends
in lattice enthalpies.
One reason is that the
lattice
arrangement of ions might not be the same for a particularly
series for comparison.
Another reason might be the
influence of covalent character lowering the melting point versus
more pure ionic character strengthening the electrostatic
attraction between the ions.
For the same Group
1 or Group 2 metal ion, there are more consistent trends e.g.
generally speaking the melting point trend is:
melting point
of fluoride > chloride > bromide > iodide (> astatide)
This fits in
with increasing anion radius reducing the Coulombic
electrostatic force, so reducing the inter-ionic bonding
force, and also fits in with decreasing ionic character,
For the same
cation you might expect the oxides, with a smaller ionic radius
to be higher melting than the corresponding sulfide, and this is
true in most cases.
So, you would
expect the melting point of oxide > sulfide for a given
Group 1 or Group 2 cation and the trend is reasonably
consistent.
